Cardiovascular System 2021-22 PDF

Cardiovascular system  the atrioventricular (AV) valves and the semilunar valves. The closure of the valves causes the “lub- dub” associated with the heartbeat. The first heart sound, or the “lub,” occurs when the ventricles contract and the AV valves close. The second heart sound,...

Cardiovascular system  the atrioventricular (AV) valves and the semilunar valves. The closure of the valves causes the “lub- dub” associated with the heartbeat. The first heart sound, or the “lub,” occurs when the ventricles contract and the AV valves close. The second heart sound, or the “dub,” occurs when the ventricles relax and the semilunar valves close.  Sinoatrial (SA) node During Phase 4, the pacemaker potential, the cells of the SA node depolarize toward threshold due to the influx of Na+ ions and Ca++ ions. The upward swing of the action potential, Phase 0, results from the influx of calcium through slow Ca++ channels. Repolarization, Phase 3, is due to the efflux of K+ ions.  Ventricular muscle The resting membrane potential, Phase 4, is very negative due to the high permeability of the K+ channels. The upward swing of the action potential, Phase 0, results from the rapid influx of sodium through fast Na+ channels. The brief repolarization that occurs during Phase 1 is due to the abrupt closure of these channels. The plateau of the action potential, Phase 2, results from the influx of calcium through slow Ca++ channels. Finally, repolarization, Phase 3, is due to the efflux of K+ ions.  Cardiac cycle 1. Systole: Where the chambers contract and eject the blood out of them. 2. Diastole: Where the chambers relax, allowing them to fill with blood. Ventricular filling (diastole) Isovolumetric contraction (systole) (first heart sound “lub”) Ejection (systole) Isovolumetric relaxation (diastole) (second heart sound “dub”)  Cardiac output Cardiac Output = Heart Rate x Stroke Volume l/min bpm mL In a healthy adult this is 70 x 75mL= 5L/min Factors that influence cardiac output Stroke volume – the amount of blood ejected from each ventricle at each heartbeat. Heart Rate – regulated by the autonomic nervous system (ANS)  Stroke Volume Stroke volume is determined by three factors: Preload Afterload Contractility  Preload - related to the volume of blood in the ventricle at the end of diastole: - End Diastolic Volume Factors affecting preload are: Blood volume Venous return The vasomotor tone Preload: Factors which influence Venous return to the heart Muscle pump Respiratory Pump Valves in the vein  ‘Starling’s law of the heart’ If, for any reason, the blood returning to the heart increases, more ventricular filling will take place during diastole; The ventricular muscle fibers will be stretched and on the next systole (contraction) it will give more powerful contraction, resulting in stroke volume. more fibers are stretched, the greater the force of the contraction (within reason)  Afterload Described as the resistance against which the ventricle must work. Whilst Preload is a major determinant of myocardial contractile power Afterload is mainly mechanical factor that affects performance  Total Peripheral Resistance (TPR) Peripheral Vascular Resistance – resistance exerted by the action of the walls of the resistance vessels impeding blood flow most resistance is provided systemically by the arterioles,and small and medium sized arteries – – exert a powerful influence in the control of blood pressure. Factors which influence TPR The length of the vessel the blood The diameter of the lumen of the vessel The viscosity of the blood  What is Blood Pressure? Blood pressure is the force exerted on a blood vessel wall by the blood. Blood must circulate through the body and organs to maintain life The Heart is the pump that circulates the blood Pressure difference in the vascular system ensures that blood flows around the body  Factors which influence blood pressure Cardiac output (CO) Total Peripheral Resistance (TPR) Or BP = CO x TPR Factors which influence BP Cardiac output (and venous return) Blood volume and viscosity Peripheral resistance Elasticity of the blood vessels